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首页> 外文期刊>Electrical Engineers - Part II: Power Engineering, Journal of the Institution of >The use of protective multiple earthing and earth-leakage circuit-breakers in rural areas (Second report)
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The use of protective multiple earthing and earth-leakage circuit-breakers in rural areas (Second report)

机译：在农村地区使用多重接地保护和漏电断路器（第二份报告）

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Fault resistance is an important factor in protection, and was not dealt with fully in the first Report Ref. F/T102. A more thorough study of this aspect of earthing has revealed very interesting results, the principal one of which is that, with a properly constructed protective multiple earthing system, the occasions when a dangerous shock may be obtained are few, and that in the majority of these cases the earth fault responsible for the liability to shock develops such a large amount of heat energy that the existence of a defective condition is likely to be revealed in some much more obvious way than by a dangerous potential on the metalwork. The maximum voltage which can occur between metalwork and earth is limited to the product of the fault current and the earth resistance, and the limiting value of the latter is 2 ohms with a P.M.E. system. Actually in the majority of cases it will be appreciably less than this. The effect of fault resistance is to reduce the current and consequently the risk of shock?at the same time, however, it may prolong the period for which the defective condition is maintained, but analysis shows that up to an appreciable value of fault resistance the energy dissipated in the fault itself is adequate either to burn it out or to draw attention to its existence, and at greater values of resistance the current is so reduced that the voltage drop across 2 ohms earth resistance is quite negligible from the shock aspect. The situation is very different from this with ordinary earthing, since there is no limitation on the earth resistance such as occurs with P.M.E. Consequently the voltage on the metalwork may have almost any value and the combined influence of the fault resistance and earth resistance may frequently be adequate to prevent the fuse blowing, and the voltage across the earth resistance may be dangerous. There are circumstances in which the fault will reveal itself by generation of heat as with P.M.E., but this does not apply ge-nnerally. With earth-leakage circuit-breakers dangerous conditions tend to occur when the fault resistances are high rather than low, since at low values the trip coil is more likely to operate. If the resistance is of the order of thousands of ohms, insufficient current may flow to trip the circuit-breaker, but sufficient current may pass through the person making contact with the metalwork to constitute an unpleasant, though probably not a dangerous, shock. Low fault resistances have no influence on the operation of the earth-leakage circuit-breaker, since this may already have several hundreds of ohms in its circuit due to the impedance of the coil and the earth electrode. The non-operation of an earth-leakage circuit-breaker due to a high-resistance earth electrode does not necessarily mean that no shock can be experienced?the two facts are comparatively unrelated since the electrode may have a high resistance because it is small or in a dry place, whilst the person touching the metalwork may be in contact with soil of quite a different resistivity, or he may be standing on a conducting floor of considerable size, in which case the soil resistivity has only a small influence. Arising from the earlier Report, and further evidence which has been secured since its issue, certain alterations have been made in the recommendations. It is now proposed that distinction be no longer made between farms and ordinary installations as regards protection, and that no links for testing or other purposes be permitted in the neutral of a P.M.E. system since there is no significance in applying any test to a neutral which is earthed at possibly hundreds of places. Modifications have been made in the previous recommendations dealing with consumers' earth electrodes and the earthing of the distributor neutral, and the opportunity has been taken to make several other less important modifications.
机译：抗故障能力是保护的重要因素，第一份报告参考中并未对此进行全面讨论。 F / T102。对接地这方面的更深入研究显示出非常有趣的结果，其主要结果是，通过适当构建的保护性多重接地系统，可能会发生危险电击的机会很少，而在大多数情况下，在这些情况下，负责电击的接地故障会产生大量的热能，因此有缺陷状况的存在可能比金属制品上的危险电位更明显地被揭示出来。金属制品和大地之间可能出现的最大电压被限制为故障电流和大地电阻的乘积，而大地电阻的极限值为2欧姆。系统。实际上，在大多数情况下，它将远小于此值。故障电阻的作用是减少电流，并因此降低电击的风险，但是，这可能会延长故障状态的维持时间，但分析表明，故障电阻的值可观。故障本身所耗散的能量足以将其烧毁或引起注意，并且在较大的电阻值下，电流如此降低，以使从电击角度来看，整个2欧姆接地电阻上的电压降可忽略不计。这种情况与普通接地情况大不相同，因为对接地电阻没有限制，例如P.M.E.因此，金属工件上的电压几乎可以具有任何值，并且故障电阻和接地电阻的综合影响通常足以防止保险丝烧断，并且接地电阻两端的电压可能很危险。在某些情况下，断层会像P.M.E.一样通过发热而暴露出来，但这一般不会适用。对于漏电断路器，当故障电阻较高而不是较低时，往往会发生危险情况，因为在较低的值下，跳闸线圈更可能工作。如果电阻约为数千欧姆，则可能没有足够的电流使断路器跳闸，但是有足够的电流可能流经与金属制品接触的人员，从而构成了令人不愉快的（尽管可能不是危险的）电击。低的故障电阻不会影响漏电断路器的运行，因为由于线圈和接地电极的阻抗，漏电断路器的电路中可能已经有数百欧姆的电阻。高电阻接地电极导致漏电断路器不工作并不一定意味着不会遭受电击？这两个事实相对无关，因为电极可能很小，因为它可能具有高电阻。在干燥的地方，接触金属制品的人可能与电阻率完全不同的土壤接触，或者他可能站在相当大的导电地板上，在这种情况下，土壤电阻率的影响很小。由于先前的报告以及自发布以来获得的进一步证据，建议中进行了某些更改。现在建议，在保护方面，不再在农场和普通设施之间进行区分，并且不允许在P.M.E.中立进行测试或其他目的的链接。由于对可能在数百个地方接地的中性线进行任何测试都没有意义，因此没有意义。在先前有关消费者接地电极和配电箱中性点接地的建议中，已经进行了修改，并且有机会进行了其他一些次要的修改。

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